1. Field of the Invention
The present invention relates to a driving circuit for a current-control-type light emitting element in which emission luminance is controlled by a current flowing through the element.
2. Description of the Related Art
In a recent situation in which attention has been paid, for example, to self light emitting displays using light emitting elements, the application and development of organic electroluminescent (EL) elements, serving as current-control-type light emitting elements in which emission luminance is controlled by a current flowing through each element, have drawn great interest, and many proposals have been made for driving circuits for such elements. In such driving circuits, it is necessary to supply, precisely, each light emitting element with a desired current. The situation is the same for driving circuits for current-control-type light emitting elements other than driving circuits for organic EL elements.
FIG. 17 is a schematic diagram illustrating a monochromatic image display panel in which light emitting elements are used in an image display unit and arranged on a two-dimensional plane. On an image display unit 4 are arranged x×y current supply circuits 1, each including a light emitting element. Accordingly, the number of horizontal pixels is x, and the number of vertical pixels is y. Column-driving control circuits 2i-2x are connected to corresponding current supply circuits (columns), and each of column driving signals Ai-Ax sets an injection current for controlling a desired amount of light emission in a corresponding current supply circuit 1. Row-selection-signal generation units 3i-3y output row control signals Bi-By, each for controlling a selection circuit included in the current supply circuit 1 of the corresponding row to which an output signal is input, so that an operation of setting an injection current in a corresponding one of the column-driving control circuits 2i-2x is always performed only for one pixel. The number of the column driving signals Ai-Ax and the number of the row control signals Bi-By may be at least one.
(Conventional Example 1 of the Current Supply Circuit 1)
FIG. 14 illustrates a current supply circuit 1a, serving as a current supply circuit included in a driving circuit for a light emitting element. The source terminal M3S (a source terminal is represented by a subscript suffix S in this specification) of a p-type transistor M3, serving as a transistor for supplying current, is connected to a power supply VCC, and a capacitor C1 is connected between the gate terminal M3G (a gate terminal is represented by a subscript suffix G in this specification) of the p-type transistor M3 and the power supply VCC. The drain terminal M3D (a drain terminal is represented by a suffix D in this specification) of the p-type transistor M3 is connected to a first terminal of a light emitting element EL. A second terminal of the light emitting element EL is grounded (GND). The gate terminal M3G is connected to the drain terminal M1D of a transistor M1, serving as a control switch for controlling a gate-terminal voltage. A control voltage Vd for setting a current value of the transistor M3 is input to the source terminal M1S of the transistor M1, and a control signal S7 is input to the gate terminal M1G of the transistor M1. In the case of FIG. 17, the column driving signals Ai-Ax correspond to the control voltage Vd, the row control signals Bi-By correspond to the control signal S7. When the control signal S7=L, the transistor M1=ON, so that the capacitor C1 is charged by the control voltage Vd, and the transistor M3 causes the light emitting element to emit light by injecting a current by a gate-terminal voltage Vg (=Vd). When S7=H, the transistor M1=OFF, so that the gate terminal M3G is held to the gate-terminal voltage Vg, and the light-emitting element continues to emit light by the gate-terminal voltage Vg. Each of the transistors M3 and M1 comprises a thin-film transistor (TFT), and the capacitor C1 is also formed according to a thin-film forming process. The capacitor C1 may comprise a parasitic capacitance of the transistors M3 and M1.
(Conventional Example 2 of the Current Supply Circuit 1)
FIG. 15 illustrates a current supply circuit 1b, serving as a current supply circuit included in a driving circuit for a light emitting element EL. The current supply circuit 1b differs from the current supply circuit 1a in the following points. The gate terminal M25G of a p-type transistor M25 having the same current driving characteristics as those of the transistor M3 is connected to the gate terminal M3G the transistor M3. The source terminal M25S of the transistor M25 is connected to a power supply VCC. The drain terminal M25G of the transistor M25 is connected to the source terminal M26S of a transistor M26. The drain terminal 26D of the transistor M26 is connected to the gate terminal 25G. A control signal S8 is input to the gate terminal M26G of the transistor M26. The drain terminal M1D of a transistor M1 is connected to the source terminal M26S. A control current Id for setting the amount of light emission is input to the source terminal M1S of the transistor M1, and a control signal S7 is input to the gate terminal M1G of the transistor M1. In the case of FIG. 17, the column driving signals Ai-Ax correspond to the control current Id, and the row control signals Bi-By correspond to the control signals S8 and S7. When S7=L and S8=L, the transistor M1=ON and the transistor M26=ON, so that a current mirror circuit consisting of the transistors M25 and M3 is obtained. At that time, when the control current Id is supplied, the current Id flows in the transistor M25, so that the voltage of the gate terminal M3G is determined by the current driving characteristics of the transistor M25, the capacitor C1 is charged to the voltage of the gate terminal M3G, and a current relating to the control current Id flows in the transistor M3 to cause the light emitting element to emit light by current injection. When S7=H and S8=H, the transistor M1=OFF and the transistor M26=OFF, so that the charged voltage of the capacitor C1 is held, a current relating to the control current Ld flows in the transistor M3, and the light emitting element continues light emission in a set state. Each of the transistors M3, M1, M25 and M26 comprises a thin-film transistor (TFT), and the capacitor C1 is also formed according to a thin-film forming process. The capacitor C1 may comprise a parasitic capacitance of the transistors M3, M25 and M26.
(Conventional Example 3 of the Current Supply Circuit 1)
FIG. 16 illustrates a current supply circuit 1c, serving as a current supply circuit included in a driving circuit for a light emitting element. The current supply circuit 1c differs from the current supply circuit 1b in the following points. The gate terminal M3G of the transistor M3 is connected to the drain terminal M26D of the transistor M26. The drain terminal M3D of the transistor M3 is connected to the source terminal M26S of the transistor M26. A control signal S8 is input to the gate terminal M26G of the transistor M26. The drain terminal M3D is connected to the source terminal M27S of a transistor M27. The drain terminal M27D of the transistor M27 is connected to a first terminal of the light emitting element, and a control signal S9 is input to the gate terminal M27G of the transistor M27. In the case of FIG. 17, the column driving signals Ai-Ax correspond to the control current Id, and the row control signals Bi-By correspond to the control signals S7, S8 and S9. When S7=L, S8=L and S9=H, the transistor M1=ON, the transistor M26=ON and the transistor M27=OFF, so that the transistor M3 operates as a bias voltage circuit receiving the control current Ld, and the light emission of the light emitting element is turned off. The capacitor C1 is charged to the voltage of the gate terminal M3G determined by the current driving characteristics of the transistor M3. When S1=H, S8=H and S9=L, the transistor M1=OFF, the transistor M26=OFF and the transistor M27=OFF, so that the voltage of the gate terminal M3G is held to the charged voltage of the capacitor C1, and a current relating to the control current Ld continues to flow in the transistor M3, to cause the light emitting element to emit light. Each of the transistors M1, M3, M26 and M27 comprises a thin-film transistor (TFT), and the capacitor C1 is also formed according to a thin-film forming process. The capacitor C1 may comprise a parasitic capacitance of the transistors M1, M3 and M26.
In the above-described conventional examples, each of the transistors M1, M26 and M27 may have any configuration, provided that the transistor can perform a switching operation by appropriately inputting a corresponding one of the control signals S7, S8 and S9. An n-type transistor may also be used instead of each of the p-type transistors M3 and M25 if connection to the light emitting element, the power supply VCC, the GND and the like is appropriately changed.
FIGS. 18A-18F show time charts, each illustrating an operation of the image display panel shown in FIG. 17. FIG. 18A indicates a control signal S(n) for the n-th row. In order to simplify explanation, it is assumed that the current supply circuits 1 for the n-th row assume a mode of setting an injection current Ir(n) for the n-th row at an L level. During a period T(n), the row control signal S(n)=L, and as shown in FIG. 18C, a corresponding one of the current supply circuits 1 for the n-th row assumes a setting mode for causing the injection current Ir(n) to flow in the corresponding light emitting element. When the the period T(n) has elapsed, the row control signal S(n) changes to an H level, and the current supply circuit 1 for the n-th row continues to cause the injection current Ir(n) to flow in the light emitting element. When an allowance period Ta(n) has elapsed, then during a period T(n+1), as shown in FIG. 18B, the row control signal S(n+1)=L, and, as shown in FIG. 18D, a corresponding one of the current supply circuits 1 for the (n+1)-th row assumes a setting mode for causing an injection current Ir(n+1) to flow in the corresponding light emitting element. When the period (n+1) has elapsed, the row control signal S(n+1) changes to the H level, and the current supply circuit 1 for the n-th line continues to cause the injection current Ir(n+1) to flow in the light emitting element.
However, the above-described current supply circuits 1a-1c are not without problems.
For example, in conventional example 1, the amount of light emission in the respective current supply circuits 1a of the image display unit in which TFT's are arranged on a large area varies due to variations in the current driving characteristics, mainly Vth, of the transistor M3, resulting in incapability of reproducing a stable image on the display panel.
In conventional examples 2 and 3, the above-described problem of variations is improved by driving the supply transistor by the gate-terminal voltage obtained by causing the control current Id to flow. However, since the Vds when setting a current by the control current Id and the Vds when holding light emission (for example, in the case of the current supply circuit 26, the Vds of the transistor M25 when setting a current and the Vds of the transistor M3 when holding light emission) differ, the flow of the same current as Id in the transistor M3 cannot be guaranteed due to the Early effect.
Furthermore, it is necessary to set the voltage value of the power supply VCC with a large margin. Consequently, the influence of variations (longer than the frame period) of the power supply voltage VCC is also present, and the reproduction of a stable image cannot be guaranteed. For the following reasons it is necessary to set the voltage value of the power supply VCC with a large margin.
(Reason 1)
The transistor M3 must be operated in a region other than a triode-characteristic region (Vds<(Vgs−Vth)) where the current driving characteristics largely vary depending on the drain-source voltage Vds. That is, the transistor M3 must be operated at least in a pentode-characteristic region (Vds>(Vgs−Vth)). Accordingly, there is a limitation in the Vds of the transistor M3, and the voltage of the power supply VCC must be larger than the operating voltage of the light emitting element.
(Reason 2)
Even if the transistor M3 is operated in the pentode-characteristic region, a larger Vds is required for the transistor M3 in order to prevent the Early effect in which the current driving characteristics largely vary depending on the value of the Vds. Accordingly, a further larger value is required for the voltage of the power supply VCC.
(Reason 3)
Organic EL elements are degraded as the accumulated value of light emission increases, and the operational voltage of light emission tends to increase. Accordingly, the voltage of the power supply voltage VCC must be still further larger.
Since the voltage of the power supply VCC must be considerably larger than the operational voltage of light emitting elements, the heat generated due to the power consumption of the TFT circuits is transmitted to light emitting elements disposed near (above or below, or to the left of or to the right of) the TFT circuits, resulting in accelerated degradation of organic EL elements which are not heat resistant.